Opal Identification Guide

What Opal Looks Like

Opal is a hydrated, amorphous form of silica (SiO2·nH2O) containing several percent water. Precious opal displays play-of-color — flashing patches of spectral colors that shift as the stone moves — caused by diffraction from an ordered array of microscopic silica spheres. Common opal (potch) has no play-of-color and ranges from milky white, gray, and blue to yellow, pink, green, and brown, with a waxy to resinous or glassy luster. Body color may be white, black, or transparent (crystal opal), and fire opal is orange-red.

• Color: any body color; precious opal flashes spectral play-of-color
• Luster: vitreous, waxy, to resinous
• Transparency: transparent (crystal/jelly opal) to opaque (potch)
• Texture: amorphous, no crystals, no cleavage; smooth conchoidal break

Step-by-Step Field ID Checklist

1. Look for play-of-color: tilt the stone — true precious opal flashes patches of color that move and change, not just a surface sheen.
2. Check hardness: opal is relatively soft (Mohs ~5.5–6.5) — it will be scratched by quartz and a hard steel file; it barely scratches glass.
3. Feel the weight: opal has low density (~2.0–2.2 g/cm³) and feels light for its size.
4. Examine luster and translucency: waxy-to-glassy, often translucent.
5. Confirm no crystal form or cleavage: amorphous, conchoidal fracture.
6. Watch for crazing: natural opal may show fine surface cracks; some hydrophane opal sticks to the tongue.

Key Diagnostic Tests

• Hardness: ~5.5–6.5; softer than quartz — useful against silica imitations.
• Density: low (~2.0–2.2); lighter than glass and quartz — a good separator.
• Streak: white.
• Cleavage/fracture: none; conchoidal.
• Acid: no reaction (silica).
• Hydrophane test: some natural opal (Ethiopian) absorbs water and may stick to the tongue or temporarily change clarity.

Common Look-Alikes and How to Tell Them Apart

• Opalite / opalized glass: manmade glass with a milky blue glow but no true play-of-color (only a flat sheen and orange backlight); often shows bubbles and is harder. Genuine opal shows moving spectral flashes.
• Synthetic opal (e.g., Gilson): shows very regular, columnar "chicken-wire"/snakeskin color patches and an unnaturally ordered pattern under magnification, unlike the irregular play of natural opal.
• Doublets/triplets: opal slice glued to a dark backing and/or capped with glass/quartz; check the side profile for a flat join line and the dome for a glassy cap.
• Labradorite/moonstone: feldspars show schiller/adularescence (single-color sheen) not multicolor play-of-color, are harder (~6), and have cleavage.
• Fire opal vs orange chalcedony/glass: fire opal is low density and softer; chalcedony is harder (~7).

Where Opal Is Found

Opal precipitates from silica-rich waters in cavities and weathered host rocks. Australia is the dominant source of precious opal (Coober Pedy, Lightning Ridge for black opal, Andamooka, Mintabie, Queensland boulder opal). Ethiopia (Welo/Wegel Tena) produces hydrophane precious opal; Mexico is famous for fire opal; Brazil, Honduras, Nevada/Idaho (USA), Peru (blue/pink common opal), and Indonesia also produce opal. In the field, look for opal in cracks and nodules within weathered sedimentary rocks and volcanic settings.